Doh!! Memory failure - thanks.
Yes, the external Laycock de Normanville OD can be used in intermediate gears, there is a chance that 3rd + OD is mid-way (or at least useful) between 3rd and 4th to reduce your 3-4 shift RPM drop (4th to OD will slow the engine down too much). If it's less than the 26% loss you have now it will help, but not much, the most common is .80, so you'd only get another 400 RPM. It may also help between 2nd & 3rd.
It's an ambitious project in itself, since it involves shortening the driveshaft, re-angling the pinion stub and/or engine height, new brackets to support the OD box?, floor mods. however: it will make a taller (4.22:1) gear streetable again.
An Eaton of the correct size can be had and installed for less than $500...
Rough sizing: Eaton turns about 2 × engine speed for best efficiency, so Eaton nominal size can be as small as 50% of engine size: a 1500cc engine can use an M45 (= 45", 737cc) at 2 × speed to get roughly 1 bar.